A. Field of Invention
The invention relates to respiratory systems such as Non-Invasive Positive Pressure Ventilation (NIPPV), nasal Continuous Positive Airway Pressure (CPAP) and other similar apparatus, used, for example, in the treatment of Sleep Disordered Breathing (SDB) or Obstructive Sleep Apnea (OSA). More particularly, this invention pertains to a respiratory apparatus which uses an automatic baseline tracking technique to monitor and display a patient's respiration, for example during a CPAP titration session of a sleep investigation.
B. Description of the Prior Art
CPAP, NIPPV and similar types of respiratory apparatus function to supply clean breathable gas (usually air, with or without supplemental oxygen) at a prescribed pressure or pressures, synchronously with a patient's respiration. A suitable CPAP apparatus in which the present invention may be incorporated is, for example, the Sullivan® V made by ResMed Ltd. of North Ryde NSW, Australia.
A respiratory apparatus typically includes a blower, an air filter, a mask or other similar patient interface, an air delivery conduit connecting the blower to the mask, and a microprocessor-based control unit. The blower generally includes a servo-controlled motor and an impeller and is used to provide a flow of pressurized air to the patient. The blower may also include a valve for discharging air. Optionally, the apparatus may include a humidifier which can apply moisture to the air supplied through the air delivery conduit. The control unit is used to control the functions of the blower, and to monitor clinical functions and other parameters associated with respiration. These parameters may be used for the diagnosis of sleep and respiratory disorders. Respiratory disorders such as apnea, snoring, and partial airflow limitations can be inferred by a clinician from the patient's respiration, the associated breathing pattern and signals from other sensors.
A convenient, established way of monitoring respiration during the diagnosis of a sleep disorder consists of analyzing pressure fluctuations obtained from nasal oxygen cannulae inserted into the patient's nares. This provides an indication of respiration flow. If upper-airway irregularities of a significant number are recorded, a CPAP titration session may be ordered. The goal of a CPAP titration session is to determine what level of CPAP treatment is needed to abolish the bulk of the patient's upper-airway irregularities. Throughout the session the CPAP level (pressure) is manually adjusted to resolve the irregularities. During such a session, respiration may be assessed by interpreting the mask pressure signal, a complex pressure signal consisting of the following components: (a) a CPAP component related to the positive airway pressure induced by the blower and having a very low frequency (in the order of 0-0.1 Hz) and high amplitude (in the order of 2-30 cm H2O); (b) a respiration component related to the normal respiration of the patient and having a relatively low frequency (of about 0.01 Hz) and low amplitude, generally not exceeding 10 mm of H2O; and possibly (c) a component associated with snoring and having a high frequency in the range of 30-200 Hz and a low amplitude in the order of mm of H2O. For diagnostic purposes, it is desirable to generate an output signal indicative of the last two components (b) and (c) to derive the respiration sequence referred to herein as the respiration signal.
Prior art respiration monitoring systems use high-pass filtering to separate the desired components from the complex pressure signal. This technique can be unsatisfactory because: (1) if the high-pass filter excludes low-frequency components of the respiratory signal, it will compromise the integrity of the monitored signal; (2) it is slow to track changes in CPAP component, particularly fluctuations due to leaks which are often known to cause step changes in the pressure signal; and (3) if performed in software, it requires high resolution and extensive signal processing.
Another known technique for deriving and monitoring a respiration signal uses a DC-coupled response amplifier without high-pass filtering of the complex pressure signal. The disadvantage of a DC-coupled technique is that the CPAP component appears as a DC offset which must be subtracted from the complex pressure signal so that the respiration signal does not exceed the dynamic range of the measurement system. During a titration study, each adjustment of the CPAP treatment pressure may demand an adjustment of the DC offset, if the respiration signal is to stay within the dynamic range of the monitoring system. Typically a special manual knob is provided for this purpose which allows an operator to eliminate the DC offset. Hence, using a DC-coupled response amplifier is time-consuming and requires a manual operation of the respiratory apparatus, additional training, and constant attention by an operator.
If the CPAP component generated by the blower is continuously known by the respiration monitoring system, an alternate technique would be to subtract this CPAP component from the complex pressure signal sensed in the mask, theoretically leaving just the respiration signal. This technique is impractical because leaks may occur, causing the pressure in the mask to deviate significantly from the pressure set for the blower and because the blower may not be in constant communication with the sensing device and, therefore, the CPAP component may not always be present.